1. Field of the Invention
This invention relates to a spread spectrum communication system, particularly to a communication method and apparatus using a frequency hopping system.
2. Description of the Related Art
The spread spectrum communication system has excellent properties such as high resistance to interference, data security, availability to high resolution range finding. Therefore, it is used in some fields like satellite communication and ground communication. In recent years, the spread spectrum communication system is progressively applied to mobile communication and local communication, etc., in expectation of such advantages as it improves utilization factor of frequency and is compatible with existing systems.
A direct spreading (DS) system and the above mentioned frequency hopping (FH) system are typical in order to realize the spread spectrum communication. Generally, the direct spreading system balancedly modulates data signals, that have gone through modulation of a carrier wave, directly by spreading codes, thereby to make an occupied bandwidth wider. The frequency hopping system switches a carrier wave for data signals into different frequencies in accordance with spreading codes, thereby to utilize a wide occupied bandwidth.
A basic structure of a conventional communication apparatus using the frequency hopping system and its communicating operation will be described referring to FIGS. 5A and 5B.
First, a transmitting operation of the conventional frequency hopping communication apparatus is shown with reference to FIG. 5A. Transmit data is processed along a route shown by a heavy solid line in the figure.
The transmit data composed of analog signals are coded in a coding circuit 1. The coded data are modulated in a modulating circuit 2 through a modulating method like a frequency shift keying (FSK) or a phase shift keying (PSK). The modulated data are mixed with outputs of a frequency synthesizer 8 in a mixer 3. The synthesized 8 changes its output frequency or carries out frequency-hopping in accordance with a spreading code series for frequency hopping system which are generated in a spreading code generating circuit 7. The spreading code series and frequency hopping will be described later. Outputs of the mixer 3 are power-amplified in a transmit circuit 4 and sent from an aerial 6 via a duplexer 5 which alternates output of the transmit data and input of receive data.
Next, the receiving operation of the communication apparatus is described referring to FIG. 5B. The receive data are processed along a route shown by a heavy solid line in the figure.
Signals received through the aerial 6 are inputted into an receive circuit 11 via the duplexer 5 and amplified through a bandpass filter of the receive circuit 11. The amplified signals are mixed with outputs of the synthesizer 8 in a mixer 10. The frequency outputted from the synthesizer 8 is hopped in accordance with spreading code series for frequency hopping system which are generated in the spreading code generating circuit 7. A synchronous circuit 9 is adapted to get and hold synchronization for the switching of the frequency in the frequency hopping system, and synchronizes the receive signals with the output frequency of the synthesizer 8. The synchronized outputs of the mixer 10 are demodulated into binary data in a demodulating circuit 13, and decoded into analog receive data in a decoding circuit 12.
The above transmit and receive operation is schematically described referring to FIG. 6. FIG. 6 illustrates a principle of the frequency hopping system. In each graph in the figure, a Y-axis shows a frequency and an X-axis shows time series.
Information signals as transmit data are modulated in the modulating circuit 2 on the basis of a predetermined modulation signals, and made into signals of frequency f4, for example. The modulated signals are spreadingly modulated through hopping frequencies which are outputted from the transmitting side synthesizer 8 in accordance with prescribed spreading codes, for example, through frequencies f5, f0, f2, f4, f6, f1 and f3. The spreadingly modulated signals are sent from the transmitting side to a receiving side. The received signals go through inverse spreading modulation through hopping frequencies which are outputted from the receiving side synthesizer 8 in accordance with receiving side spreading codes corresponding to the spreading codes at the transmitting side. Such signals are entered into the bandpass filter incorporated in the receive circuit 11, that passes only signals of a fixed frequency fn. Then, signals of a desired frequency, e.g. the frequency f4 are outputted therefrom and demodulated into signals of frequency f4 in the demodulating circuit 13 on the basis of predetermined demodulating signals corresponding to the modulating signals at the transmitting side. Thus, demodulated signals as receive data of frequency f4 are obtained corresponding to the information signals at the transmitting side.
Next, a synchronizing operation of the synchronous circuit 9 in the FH system is described in detail referring to FIGS. 7 to 10. FIG. 7 shows a relation of spreading code series and hopping frequencies allocated therefor. FIG. 8 shows a hopping operation by spreading code series of No. 1. FIG. 9 shows a relation between a detected hopping frequency pattern and received hopping frequency patterns in the synchronizing operation. FIG. 10 shows a process after detecting a trigger frequency for synchronization.
As mentioned above, the FH system carries out the modulation of the transmit data by use of the carrier frequencies generated in the spreading code generating circuit 7 at the transmitting side, e.g. the frequencies f7, f6 ,f5, f2, f4, f1, f3 that are hopped in accordance with the spreading code series No.1 for FH system in FIG. 7. Such transmit data are sent from the aerial 6. The relation between the hopped carrier frequencies f7, f6 ,f5, f2, f4, f1, f3 and the time series is shown in FIG. 8.
The receiving side communication apparatus detects received waves through the carrier frequencies f7, f6 ,f5, f2, f4, f1, f3 that are hopped in accordance with the spreading code series No.1 for FH system in the spreading code generating circuit 7. The receiving side apparatus continue detection in accordance with such pattern of hopping frequencies. However, such pattern of frequencies at the receiving side is not synchronous with a pattern of carrier hopping frequencies sent from the transmitting side apparatus. Therefore, to begin with, it is necessary to synchronize the former with the latter.
The synchronization is completed when the frequency hopping pattern of the received waves at the receiving apparatus coincides perfectly with the hopping pattern generated therein over one hopping cycle. If even one hopping frequency in the patterns differs, it means that the synchronization was unsuccessful.
Then, the synchronization is performed as follows.
First, the receiving side apparatus selects a frequency at random among the hopping frequencies of the prescribed pattern No. 1, e.g. the frequency f7, through the synchronous circuit 9. Then, the apparatus activates the routine shown in the flowchart of FIG. 10 in accordance with the detected wave pattern of FIG. 9.
The receiving side apparatus finishes a preliminary synchronization when all the frequencies f6 and f5 following the frequency f7 completely coincide between the detected wave and the received wave in steps S11 and S12. After finishing the preliminary synchronization, assuming that the detected waves and received waves are synchronized for a time being, the apparatus continues modulating provisionally with such timing in a step S14. Then, the received waves coming thereafter are continuously detected with the following hopping frequencies f2, f4, f1 and f3 of the pattern No. 1 in steps S15 to S18.
For example, the receiving side apparatus does not determine that the synchronization has been completed until the received hopping frequencies coincide perfectly with the detected frequencies over one hopping cycle after the preliminary synchronization, as shown by the received wave patter (a) of FIG. 9. Then, it activates a synchronization holding process in a step S19 so that the obtained synchronization is maintained.
As shown by a received wave pattern (b) of FIG. 9, if even one hopping frequency is not synchronous with one of the detected wave pattern for one hopping cycle during detection after the preliminary synchronization, the apparatus determines that the preliminary synchronization is unsuccessful at that moment. In this case, a frequency f0 of the received waves (b) does not coincide with a frequency f3 of the detected waves in a period T7, so that the apparatus stops modulation that has been started and exits the routine in a step S20.
As shown by a received wave pattern (c) of FIG. 9, if even one hopping frequency does not coincide with one of the detected wave pattern in the first three hopping periods t1-t3 as the preliminary synchronization periods, the apparatus determines that the preliminary synchronization is unsuccessful at that moment. In this case, a frequency f2 of the received waves (b) does not coincide with a frequency f5 of the detected waves in a period T3, so that the apparatus exits the routine in the step S20, as in the case of the received pattern (b).
As a matter of course, as shown by a received wave pattern (d) of FIG. 9, if the apparatus does not detect the frequency f7 as a trigger for synchronization for the hopping frequency pattern No. 1 of FIG. 7, the routine of FIG. 10 is not called and the synchronizing operation is not activated.
Such process is a generally used synchronizing system for the FH system and well-known.
As mentioned above, the conventional FH communication apparatus successively varies the carrier waves for spreading. It means a narrow-band transmission if seen at each period of the frequency hopping cycle. Accordingly, when a carrier frequency of this apparatus coincide with that of existing communication waves or some narrow-band jamming waves or a hopping frequency of other FH communication apparatus, the transmit signals on that frequency are error signals. For example, interference waves may coexist in a frequency shown by cross-hatching in the third graph from the left of FIG. 6. Then, such interference waves will coexist in the portions, shown by cross-hatching in the fourth and last graphs, of desired output signals passed through the bandpass filter after inverse spreading process, so that the output signals are erred. Moreover, the conventional apparatus may not be able to receive signals of a certain frequency band due to multipass phasing.
Thus, an error correction technique is necessary, and the coding circuit 1 and the decoding circuit 12 are added with another coding and decoding functions, respectively, for correcting errors. Such addition of the error correction function causes data to have redundancy in data transmission. Provided that a transmission rate is constant, throughput for data is lowered to a certain degree in order to secure transmission quality. Then, it is important what level the error correction capacity is set to. Actually, if such capacity is set very large, the data throughput is lowered accordingly. Therefore, the fact is that the error correction capacity is decided supposing an error rate in radio wave propagation. If the error correction capacity is below the supposed error rate, it causes errors.
These days, the spread spectrum communication is expected to be used in various frequency bands, places and conditions, so it is assumed that radio wave condition differs to a great extent depending on service places. Namely, it is supposed that, if only a uniform or standard error correction function is provided, communication quality cannot be maintained sufficiently in some of various service forms. Moreover, it is clear beforehand that many errors are produced in a data transmission of a specific frequency band. Therefore, it is undesirable transmit data with hopping frequencies including such frequency band, in view of its transmission efficiency, transmission rate and communication quality.